This invention relates generally to a highly active iron oxide dry sorbent for the oxides of sulfur contained in gases, and more particularly, to a procedure for preparing such active sorbent. The invention also relates to the use of such oxides as sorbents.
Many industrial operations produce iron sulfate, sulfur-bearing gases, or combinations of both as waste materials. Such waste materials are commonly produced from mining and industrial operations involving the production of non-ferrous metals, coal, steel, titanium pigments, sulfuric acid, elemental sulfur, electric power, and similar products. Discharge of these waste materials results in undesirable pollution of air and water.
Iron sulfate has been used, for example, in the treatment of sewage and the making of iron oxide pigments, but the production of this by-product has been greater than its consumption in such processes. Therefore, where iron sulfate is produced in significant quantities, it has been necessary to dispose of the material as a waste material such as by dumping on land or in water.
A number of attempts have been made to convert the sulfate to some more useable form of iron-bearing material such as iron oxide. For example, U.S. Pat. No. 3,195,981 describes the decomposition of iron sulfate at temperatures between 700.degree. and 1100.degree. C. Decomposition at the lower temperature is satisfactory so long as the oxygen content in the gases leaving the bed is 3% or less, otherwise SO.sub.3 is formed. The decomposition of metal sulfates, particularly mixtures of metal sulfates also is described in U.S. Pat. No. 3,630,943. Regeneration of manganous oxide from a manganese sulfate spent sorbent in the presence of reducing agents is described in U.S. Pat. No. 3,723,598. Manganous oxide is preferred over manganese dioxide as an absorbent.
Air pollution with sulfur dioxide is a major problem in the United States today. Sulfur dioxide is objectionable principally because above relatively low concentrations it is toxic to human beings and animals and is destructive to vegetation. Sulfur dioxide and its oxidation products, sulfur trioxide and sulfuric acid, are a major source of acidity in rain and fog which in turn can be very corrosive.
At the present time, the largest amount of industrial sulfur oxide emissions results from the combustion of certain types of coal and oil which contain appreciable amounts of sulfur. Waste gas streams containing sulfur dioxide similarly are produced by other industrial processes such as in the smelting of sulfur-bearing ores, the refining of sulfur-containing crude oils, the synthesis of sulfuric acid, the sulfonation of hydrocarbons, the production of coke, the production of sulfur in a Claus process, the production of paper by way of wood-pulping process, and similar industrial processes.
Furthermore, the discharge of these gas streams containing sulfur dioxide into the atmosphere constitutes a waste of a valuable material because the sulfur contained therein is an industrial commodity. Currently, tens of millions of tons of sulfur oxides are released into the atmosphere over populated regions of the United States each year. Thus, the recovery of some of this sulfur dioxide either as such or in another form could result in the accumulation of a supply of useful chemicals of definite value.
Many processes have been proposed for removal of sulfur dioxide from these gas streams. Most of the proposed removal procedures which have been suggested utilize liquid sorption in which the sulfur dioxide containing gases are intimately contacted with an aqueous sorbent which typically contains chemicals in solution or in slurry which will react with the sulfur dioxide and absorb the same into the liquid solution. Examples of such sorbents include the oxides, hydroxides and carbonates of ammonia, the alkali metals, and the alkaline earth metals.
One disadvantage of the wet sorption process is that the sorption of the sulfur dioxide must occur at a rather low temperature. This results in cooling of the gases which are ultimately discharged to the atmosphere. Such cool gases will remain near ground level thus causing pollution of the ambient air at ground level which may be as serious as that presented by the untreated flue gas.
Other methods have been suggested for removing sulfur oxides from flue gases. Attempts to desulfurize fuels prior to combustion have been costly and not always effective. For some fuels, such as coal, many processes investigated to date do not economically desulfurize fuel.
Additive processes have been suggested wherein materials having the ability to combine with sulfur oxides are added either to the fuel or to the combustion gases. Additives which have been employed include soda, limestone, magnesia and magnesite, but such additives generally are costly.
Dry adsorption also has been suggested. Sulfur dioxide can be adsorbed at low temperatures by materials such as aluminum oxide, activated carbon, and silica gel. A disadvantage of such adsorption processes is that they also require relatively low temperatures and have similar drawbacks to those of the wet absorption process described above.
Solid acceptors which absorb sulfur oxides also have been reported. Examples of such acceptors include alkalized alumina which is converted to aluminum sulfate and mixtures of alkali metal oxides and iron oxide which are also converted to the corresponding sulfates. One important advantage of these solid absorption processes is that they can be operated at elevated temperatures, and the gas which ultimately is discharged to the atmosphere is at an elevated temperature and is readily dissipated in the atmosphere. There continues to be a need, however, for solid acceptors which are regenerative and economically acceptable in commercial scale absorption processes.